Speed limiting means for an air valve carburetor

ABSTRACT

An air valve upstream of a manually-controlled throttle valve, both pivotally disposed in a mixing conduit of an air valve carburetor, is linked to fuel metering means and also to a spring that biases the air valve to a normally closed position. The air valve is opened by a diaphragm-type fluid motor having a first pressure chamber communicating with a vacuum port intermediate the air valve and throttle valve through a vacuum passage and a second pressure chamber communicating with atmospheric pressure through an atmospheric passage. Below a predetermined vehicle speed, both chambers communicate continuously with their respective pressures with the result that the fluid motor positions the air valve in relation to the inducted air flow. Above this predetermined speed, the spring is allowed to close the air valve to a position where the air flow is limited to that required to maintain the predetermined speed. The rate of air valve closure is controlled by cooperation between the fluid motor and a solenoid valve intermittently operated by a vehicle speed sensor for duty cycles increasing as the actual vehicle speed exceeds the predetermined speed. The solenoid valve connects the first pressure chamber alternately between the atmospheric and vacuum passages for durations sufficient to effect a pressure increase in the pressure chamber that allows closure of the air valve at a rate to limit the vehicle speed.

United States Patent [191 Bedard et al.

[ SPEED LIMITING MEANS FOR AN AIR VALVE CARBURETOR [75] Inventors:Francis L. Bedard, Troy; Robert T.

Price, Berkley, both of Mich.

[73] Assignee: General Motors Corporation,

Detroit, Mich.

[22] Filed: Sept. 30, 1971 [21] Appl. No.: 185,283

Primary Examiner-Laurence M. Goodridge I Attorney- E. W. Christen, R. L.Phillips et'al.

[57] ABSTRACT An air valve upstream of a manually-controlled throttleAug. 28, 1973 valve, both pivotally disposed in a mixing conduit of anair valve carburetor, is linked to fuel metering means and also to aspring that biases the air valve to a normally closed position. The airvalve is opened by a disphragm-type fluid motor having a first pressurechamber communicating with a vacuum port intermediate the air valve andthrottle valve through a vacuum passage and a second pressure chambercommunicating with atmospheric pressure through an atmospheric passage.Below a predetermined vehicle speed, both chambers communicatecontinuously with their respective pressures with the result that thefluid motor positions the air valve in relation to the inducted airflow. Above "this predetermined speed, the spring is allowed to closethe air valve to a position where the air flow is limited to thatrequired to maintain the predetermined speed.

. The rate of air valve closure is controlled by cooperation between thefluid motor and a solenoid valve intermittently operated by a vehiclespeed sensor for duty cycles increasing as the actual vehicle speedexceeds the predetermined speed. The solenoid valve connects the firstpressure chamber alternately between the atmospheric and vacuum passagesfor durations su'fi'icient to effect a pressure increase in the pressurechamber that allows closure of the air valve at a rate to limit thevehicle speed.

3 Claims, 2 Drawing Figures hicle spced limitatio'ns.

ln-an air valve carburetor, the fuel flo'w is controlled in accordancewith'the air flow by maintaining a pressure differential across aluel'm'etering orifice'proportional to thatacross an'airvalve and byvarying thearea of the orifice with'the areapast'the air valve. Toeffect this control, a fuel metering rod displaceable inthe meteringorifice is linked to and movedby the'air'valve. The airvalveinturnislinked'toafluid 'motor having a pairof pressure chamberscommunicating'throug'h respective atmospheric and vacuum passages withthe pressures upstream and downstream of the air valve andoperative toopen the-air valve in relation to the pressure differentialtherea'cross. The fuel flow is thereby controlled-in accordance "withthe air flo'wto provide an air-fuel mixture suitable =forinternalcombustion.

To limit'to 'apredetermined vehicle speed the'speed of aivehicle drivenby "an air-valve-ty'pe carburetorinternal combustion engine, the presentinvention 'provides for the'ai'r valve to 'be biasedfrom its'opened position to a positiona-t which the airflow is limited "to effectthefpredetertriined speedfSuch limiting of air flow iseffccted =bycooperation between "the fluid 'rnoto'r and a solenoid 'valve. Thelatte'r isintermittently operated by a speed -sensor'=for duty cyclesincreasing-as the vehicle speed exceeds the predetermined speed. Thesolenoid valve connects a inotor pressure chamber normally-connected tothe vacuum passage alternately between the vacuum andatmosphericpassages. The durations of such connections are sufficient tocause the pressure increase effected "in the pressure chamber byconnection tothe atmospheric passage when the solenoid is deenerg'izedto just exceed the pressure decrease pull down when connected to thevacuum port and thereby control the closure rate of the air valve.

it is therefore a primary object of the present invention to provide anew and improved automotive air valve carburetor for an internalcombustion engine.

It is another primary object of the present invention to provide, in anair valve carburetor having a fluid motor for opening an air valve inaccordance with the air flow thereacross, a vehicle speed limiteroperative at a predeterminedsp'eed to limit air flow by allowing airvalve .closure.

It is another primary object of the present invention to provide, in anair valve carburetor, a fluid motor for positioning an air valve inaccordance with the air flow thereacross, the fluid motor having onepressure chambcr communicating with an atmospheric pressure passageupstream of the air valve and a second pressure chamber communicatingthrough a vacuum passage to a vacuum port downstream of the air valve,and a speed limiter operative at a predetermined speed to permit closureof the air valve by connecting the pressure chamber normally connectedwith the vacuum passage alternately to the vacuum passage and theatmospheric passage. I

It is a further and more specific object of the present invention toprovide, in an air valve carburetor of the foregoing type, a solenoidvalve intermittently operated by a speed sensor to alternately connectthe atmospheric and vacuum passages to one of the fluid motor pressurechambers to control theclo'su re rate of the air valve.

lt is a further and more specific object of the present invention tooperate a solenoid, in a speed lirniter and an air valve carburetor ofthe foregoing type, at a duty cycle that causes the pressure increase inthe pressure chamber when connectedto the atmospheric passage to'exceed'the pressure decrease when connected to the vacuum passage. I

These and other features, objects and details of the present inventionwill become more apparent with refere'nce to the following descriptiontaken in conjunction with the following drawings, wherein:

FIG. 1 is 'a cross-sectional view of an air valve carburetor having aspeed limiter constructed in accordance with the present invention; and,

'FIG. 2 is a schematic of an electromechanical speed sensor foroperating the'speed limiter in FIG. 1.

Referring now toFlG. 1, there is shown an air valve carburetor 10 forproviding an air-fuel mixture to an internal combustion engine.Carburetor 10 has a mixing conduit '12, the flow of air through which iscontrolled by a throttle valve 14 pivotally disposedin conduit l 2'andmanually operated in the customary ma'nnerfFu'el 16 is deliveredfro'm afloat bowl [8 to mixing conduit l2throug h a metering orifice 20,ap'assage 22, and a nozzle '24, at a'rate determined by the pressuredifferential across orifice '20, whose area is made vari-- able by axialdisplacement thereth'rough of a tapered meteringrod 2 5. v 7

Also pivotally, and here centrally, disposedin mixing conduit 12upstream of throttle valve 14 butterflytype a'irvalve 26 that is biasedby alight pr'ing 29 t close conduit 12 when the pressure drop acrossvalve 26 is less than a few inches of water, here about 6 inches. Links27 and 28 connect air valve 26 to metering rod 25 to increase fuel flowthrough orifice 20 by causing a leftward displacement of rod 25 as airvalve 26 opens. Another link 30 connects air valve 26 to a flexiblediaphragm 32 of a fluid motor 33 where in diaphragm 32 is spaced betweenan air valve opening pressure chamber 34 and an air valve closingpressure 'chamber 36. Opening chamber 34 communicates with atmosphericpressure through an air passage orport 38 and closing chamber 36normally communicates with a vacuum or signal port 40 locatedintermediate throttle valve 14 and air valve 26 in mixing conduit 12through a vacuum or signal passage comprising conduit 42, a vacuum pipe'44, air-vacuum chamber 46, and a conduit 48.

Located within chamber 46 is a solenoid valve 5t] comprising a coil 52on a bobbin 53 secured around a vent pipe 56 secured in a frame 54integral with and ex tending from a wall (not shown) of the carburetor.One

end 58 of pipe 56 extends through air port 38 and the other erid 60 isopposite and spaced from vacuumpip'e 44. Located between vent pipe 56and vacuum pipe 44 is a'double-faced valve element 62 which is fulcrumedat projection 64 on the frame 54. Valve element 62 is the armature ofthe solenoid and, when coil 52 isdeenergized in speed limitingoperation, is pivoted by a spring 66 to close vacuum pipe 44 and openvent pipe 56 to chamber 56 and thus to chamber 36. In nonspeed limitingoperation, coil 52 is energized so that valve element 62 normally closesvent pipe 56 and continually opens vacuum pipe 44.

Except for solenoid valve 50 and the inclusion of pipe 44 and chamber 46in the signal passage, carburetor is as shown and described in greaterdetail in U.S. Pat. No. 3,314,663, issued to Stanley H. Mick, andassigned to the assignee of the present invention.

ln normal non-speed limiting operation with the solenoid valve 50energized, the rate that air and fuel are admitted to and mixed inconduit 12 is determined by the setting of throttle valve 14 to effectthe desired vehicle performance, and the pressure drop across orifice isproportional to that across air valve 26. The fuel flow through orifice20 is controlled in accordance with the air flow past air valve 26 bycontrolling the area of orifice 20 in accordance with the air valveposition. Thus, as throttle valve 14 is opened to effect a desired levelof vehicle performance, the flow of air-fuel mixture drawn throughconduit 12 increases to momentarily increase the pressure drop acrossair valve 26. The decreased downstream signal pressure is communicatedfrom vacuum port 40 to valve opening chamber 36 with valve 62 normallyopening pipe 44. This momentarily increases the pressure differentialacting on diaphragm 32 causing a force thereon to increase, throughlinkage 30, the opening of air valve 26 and also the flow area oforifice 20. i

The spring rate and points of connection of valve closing spring 29 toair valve 26 are selected to provide a substantially constant closingtorque on the air valve 26. This torque is selected to be small enoughto be overcome by the air flow at idle operation and opposes an openingtorque resulting from the pressure differential acting on diaphragm 32in a stabilized throttle-load condition. When this pressure differentialmomentarily increases on throttle and/or load change, an opening torquegreater than the closing torque is applied to air valve 26, causing airvalve 26 to be opened to a position at which the pressure differentialthereacross drops to where the two torques again balance.

An electromechanical speed sensor apparatus 70 for operating solenoidcoil 52 is shown in FIG. 2 and here includes an input shaft 72 connectedto be driven by a vehicle wheel (not shown) as by way of a flexiblecable (not shown). An eccentric cam 73 is mounted on shaft 72 to impartoscillatory movement to a cam follower 74 pivoted on stationary andelectrically isolated pins 75. At the other end of cam follower 74 is acontact arm 77 carrying an arcuate-shaped contact 76 for engaging acantilevered or dithering" contact 78. Such engagement is effected oncontact 78 at a point thereon intermediate one end secured to astationary support 80 and another end 81 which is free to contactspeed-control means to be described shortly.

Also secured to support 80 is a speed cup 82 containing a magnetic dragelement 84 that rotates a field plate 85, and a spindle 86 connectedthereto, in proportion to the speed of shaft 72 and against a returnforce applied by a coiled spring 88. Spindle 86 carries a speedindicator 90 for indicating speed on scale 92 and a contact interrupterpin 94 that is rotated clockwise with increasing vehicle speed.

Pin 94 is positioned on spindle 86 to commence intermittent contact withthe free end 81 at a predetermined vehicle speed as contact 78 is beingadvanced and retracted relative to spindle 86 by cam contact 76. Wheninterrupter pin 94 beings to intermittently contact dithering contact78, the retraction thereof to follow cam contact 76 is prevented so thatcam contact 76 leaves dithering contact78 and. does not resumev contactagain until the retum advance stroke. Further clockwise rotation'ofinterrupter pin 94 with increasing vehicle speed increases theproportion of the advancement and retraction oscillation period of camfollower 74 during which cam contact 76 leaves dithering contact 78.Except for the cooperation with fluid motor 33 and the electricalcircuit to-be described below, speed sensor is as shown in greaterdetailin U.S. Pat. No. 3,601,104, issued Aug. 24, 1971, issued to Mark N.Culver and assigned to the assignee of the present in vention.

The above-described interruption of mechanical contact between contacts76 and 78 is used to deenergize solenoid coil 52, one end of which isconnected to the positive terminal of vehicle battery 100 through theclosure of a vehicle ignition switch [02. The negative terminal ofbattery 100 is connected to a point of constant reference potential suchas support or ground 80. The other end of solenoid coil 52 is connectedto the emitter of a PNP transistor 104, the collector of which isgrounded at and the base of which is connected between voltage dividingresistors 106 and 108. These resistors are connected in series betweenthe positive terminal of battery and ground 80 through ignition switch102 and contacts 76 and 78.

Below the desired vehicle speed limit, contacts 76 and 78 are closed tocomplete a biasing circuit rendering transistor I04 conductive andcausing sufficient emitterto collector current to flow therethrough toenergize coil 52. As interrupter pin 94 enters the range of oscillatorymotion of dithering contact 78 to interrupt contact with cam contact 76,as described above, the transistor 104 biasing path is interrupted forshort periods during which current flow through transistor 104 and coil52 is also interrupted. To protect transistor 104 against the effects ofreverse voltages upon interruption of current flow therethrough, a diode93 is connected from the collector to the emitter electrodes oftransistor 104.

When contact interrupter pin 94 enters the range of oscillation ofdithering contact 74,-as has hereinabove been described, port 60 isopened and closed-according to the operation of armature 62 attendantthe energization and deenergization of coil 52. The periods that port 60is opened are very short as pin 94 just enters the range of ditheringcontact 78 compared to the periods that port 60 is closed. This ratio,or duty cycle, between the opened and closed periods of the port 60increases as vehicle speed increases to rotate interrupter pin 94clockwise into the free end 81 of dithering contact 78. Depending on therange of advancing and retracting oscillation of the cam contact 76,a100 percent duty cycle, effecting a continual opening of port 60, may bemade to occur just above the predetermined speed limit, e. g. l or 2miles per hour above this limit. This variation in duty cycle with speedeffects a smooth transition from a non-limiting to a limiting control ofvehicle speed, and the predetermined limit is not exceeded unless thethrottle load or road load conditions are such as to effect vehicleacceleration through the desired speed limit. Only above the upper limitof this range does the limiting system allow closure of the air valve 26by the action of the spring 29 that might be rapid enough to beappreciably noticed.

The opening of vent port 60 by armature 62 allows atmospheric air topass into air vacuum chamber 46 and therefrom through conduit 48 into-valve opening chamber 36. As soon as the pressure in chamber 36 isincreased, the opening bias applied through-link 30 to air valve 26isdecreased to allow closure thereof by spring 29 to effect a reductionin air flow and, thereby vehicle speed. A smooth transition from acondition where the system does not limit the speed to a condition whereit does is effected by increasing the rate of air valve closure as theactual vehicle speed overshoots the predetermined limit. The closurerate of air valve 26 is determined by the magnitude of the torqueunbalance thereon which in turn is determined by the pressure inpressure chamber 36. However, the pressure in chamber 36 does notincrease instantaneously to atmospheric pressure when vent port 60 isopened by valve element 62. The pressure increases at a rate thatdepends on the .duty cycle and on the sizes of the passages, conduitsand chambers. The minimum rate of pressure increase is effected at apercent duty cycle when vacuum pipe 44 is continually opened by valveelement 62 and vent port 60 continually closed. As the duty cycleincreases from 0 percent where there is no speed limiting, not only doesthe quantity of atmospheric air entering chambers 46 and 36 through ventport 60 increase, but also the offsetting effect of air from vacuum port40 decreases. The maximum rateof pressure increase in chamber 36effecting a maximum closure rate for air valve 26 is therefore effectedat a 100 percent duty cycle.

Whenever the vehicle speed starts to exceed the predetermined speed at aslow rate, the response of the speed limiting system is such that a lowduty cycle provided by interrupter pin 94 just entering the ditheringrange of contact 78 is sufficient to effect a slow closure of the airvalve and a prompt return to the speed limit. As the rate that thevehicle speed overshoots the predetermined speed increases, the dutycycle is increased to compensate for the increased overshoot to whereultimately vent pipe 60 is continually open and vacuum pipe 44continually closed.

Thus, by limiting theair flow to limit the vehicle speed to thepredetermined speed, the air valve at or above the limiting speedessentially replaces the function performed by the throttle valve belowthe predetermined limiting speed. In other words, the cooperationbetween the vehicle speed sensor and the solenoid to control the rate ofair valve closure to limit vehicle speed is analogous to. thecooperation between the driver of the vehicle and, the throttle valve tolimit the speed at speeds below the predetermined limit.

,Having described one embodiment of the present invention, it isunderstood that the specific terms and examples are employed in adescriptive sense only and not for the purpose of limitation. Otherembodiments of the invention, modifications thereof, and alternativesthereto may be used. I therefore aim in the appended claims to coversuch modification and changes as fall within the true scope and spiritof my invention.

What [claim as new and desireto secure by Letters Patent of the UnitedStates is:

1. An internal combustion engine air valve type carburetor comprising:

a. a conduit for admitting and mixing air with fuel;

b. manually. controlled throttle valve for controlling the flowofair-fuel mixture to said engine;

c. anair. valve upstream of said throttle valve;

d. fuel metering means connected with said air valve for controlling theflow of fuel with the position of said air valve;

e. A first spring for uging said air valve from an open to a closedposition to close off said air-fuel flow;

f. fluid pressure operated motor means connected with said air valve andincluding a valve opening chamber and a valve closing chamber for urgingsaid air valve toward said open position when the pressure in one ofsaid opening and closing chambers increases relative to the pressure inthe other of said chambers and allowing said first spring to close saidair valve when the pressure in said one chamber decreases relative tothat in the other;

g. air passage means for communicating atmospheric pressure to one ofsaid chambers;

h. vacuum passage means for communicating. the pressure in said conduitintermediate said air valve and said throttle valve to the other of saidchambers;

. vehicle speed responsive motor control means for controlling saidfluid motor including a speed responsive valve element for closing saidvacuum passage means while connecting said air passage means to saidother chamber above a predetermined vehicle speed; and v I j. a secondspring for cooperating with said valve element to close said air passagemeans to said other chamber while opening said vacuum passage meansbelow said predetermined speed. I 2. An internal combustion engine airvalve typecar buretor comprising: I

a. a conduit for admitting and mixing air with fuel;

b. a manually controlled throttle valve for controlling the flow ofair-fuel mixture to said engine;

c. an air valve upstream of said throttle valve;

d. fuel metering means connected with said air valve for controlling theflow of fuel with the position of said air valve;

e. a spring for urging said air valve from an open to a :closed positionto close off said air-fuel flow;

f. fluid pressure operated motor means including a valve opening chamberand a valve closing chamher for urging said air valve toward said openposition when the pressure in one of said opening and closing chambersincreases relative to the pressure in the other of said chambers andallowing said spring to close said 'air'valve when the pressure in onechamber decreases relative to the pressure in the other;

g. air passage means for communicating atmospheric pressure to said oneof said chambers;

h. a vacuum passage for communicating the pressure in said conduitintermediate said air valve and said throttle valve to the other of saidchambers; a

i. vehicle speed limiting means for increasing the pressure in saidother chamber as vehicle speed increases above a predetermined speed,said limiting means including i. a valve armature element foralternately connecting said air passage means and said vacuum pas-' sageto said other chamber, ii. solenoid means operable between energized anddeenergized states for causing said valve armature element to connectsaid air passage means to the other chamber when in one of said statesand to connect the vacuum passage to the other chamber when in the otherstate, and

iii. vehicle speed responsive switching circuit means for switching saidsolenoid between said states when the vehicle speed exceeds saidpredetermined speed and for increasing the duration of said one staterelative to the duration of the other state with increasing vehiclespeed past said predetermined speed.

3. An internal combustion engine air valve type carburetor comprising:

a. a conduit for admitting and mixing air with fuel;

b. a manually controlled throttle valve for controlling the flow ofair-fuel mixture to said engine;

c. an air valve upstream of said throttle valve;

d. a fuel metering valve connected with said air valve for controllingthe flow of fuel with the position of said air valve;

e. a spring for urging said air valve from an open to a closed positionto close off said air fuel flow;

f. a fluid pressure operated motor including a housing, a diaphragmseparating said housing into a valve opening chamber and a valve closingchamber and a linkage connecting said diaphragm to said air valve forurging said air valve toward said open position when the pressure in oneof said opening and closing chambers increases relative to the pressurein the other of said chambers and allowing said spring to close said airvalve when the pressure in one chamber decreases relative to thepressure in the other;

an air passage for communicating atmospheric pressure to said one ofsaid chambers;

a vacuum passage for communicating the pressure in said conduitintermediate said air valve and said throttle valve to the other of saidchambers;

. a vehicle speed responsive motor control for controlling said fluidmotor to increase the pressure in said other chamber as vehicle speedincreases above a predetermined speed, said control including a solenoidhaving a valve armature element for alternately connecting said airpassage and said vacuum passage to said other chamber, said solenoidoperable between energized and deenergized states for causing said valvearmature element to connect said air passage to the other chamber whenin said deenergized state and to connect the vacuum passage to the otherchamber when in said energized state, and vehicle speed responsiveswitching circuit means including a switch for alternately energizingand deenergizing said solenoid to switch said solenoid between saidstates, said circuit means further including a switch control mechanismresponsive to vehicle speed to control the opening and closing of saidswitch to increase the duration of said deenergized state relative tothe duration of the energized state with increasing vehicle speed pastsaid predetermined speed to effect a smooth transition of vehicle speedto said predetermined limit.

1. An internal combustion engine air valve type carburetor comprising:a. a conduit for admitting and mixing air with fuel; b. manuallycontrolled throttle valve for controlling the flow of air-fuel mixtureto said engine; c. an air valve upstream of said throttle valve; d. fuelmetering means connected with said air valve for controlling the flow offuel with the position of said air valve; e. A first spring for ugingsaid air valve from an open to a closed position to close off saidair-fuel flow; f. fluid pressure operated motor means connected withsaid air valve and including a valve opening chamber and a valve closingchamber for urging said air valve toward said open position when thepressure in one of said opening and closing chambers increases relativeto the pressure in the other of said chambers and allowing said firstspring to close said air valve when the pressure in said one chamberdecreases relative to that in the other; g. air passage means forcommunicating atmospheric pressure to onE of said chambers; h. vacuumpassage means for communicating the pressure in said conduitintermediate said air valve and said throttle valve to the other of saidchambers; i. vehicle speed responsive motor control means forcontrolling said fluid motor including a speed responsive valve elementfor closing said vacuum passage means while connecting said air passagemeans to said other chamber above a predetermined vehicle speed; and j.a second spring for cooperating with said valve element to close saidair passage means to said other chamber while opening said vacuumpassage means below said predetermined speed.
 2. An internal combustionengine air valve type carburetor comprising: a. a conduit for admittingand mixing air with fuel; b. a manually controlled throttle valve forcontrolling the flow of air-fuel mixture to said engine; c. an air valveupstream of said throttle valve; d. fuel metering means connected withsaid air valve for controlling the flow of fuel with the position ofsaid air valve; e. a spring for urging said air valve from an open to aclosed position to close off said air-fuel flow; f. fluid pressureoperated motor means including a valve opening chamber and a valveclosing chamber for urging said air valve toward said open position whenthe pressure in one of said opening and closing chambers increasesrelative to the pressure in the other of said chambers and allowing saidspring to close said air valve when the pressure in one chamberdecreases relative to the pressure in the other; g. air passage meansfor communicating atmospheric pressure to said one of said chambers; h.a vacuum passage for communicating the pressure in said conduitintermediate said air valve and said throttle valve to the other of saidchambers; i. vehicle speed limiting means for increasing the pressure insaid other chamber as vehicle speed increases above a predeterminedspeed, said limiting means including i. a valve armature element foralternately connecting said air passage means and said vacuum passage tosaid other chamber, ii. solenoid means operable between energized anddeenergized states for causing said valve armature element to connectsaid air passage means to the other chamber when in one of said statesand to connect the vacuum passage to the other chamber when in the otherstate, and iii. vehicle speed responsive switching circuit means forswitching said solenoid between said states when the vehicle speedexceeds said predetermined speed and for increasing the duration of saidone state relative to the duration of the other state with increasingvehicle speed past said predetermined speed.
 3. An internal combustionengine air valve type carburetor comprising: a. a conduit for admittingand mixing air with fuel; b. a manually controlled throttle valve forcontrolling the flow of air-fuel mixture to said engine; c. an air valveupstream of said throttle valve; d. a fuel metering valve connected withsaid air valve for controlling the flow of fuel with the position ofsaid air valve; e. a spring for urging said air valve from an open to aclosed position to close off said air fuel flow; f. a fluid pressureoperated motor including a housing, a diaphragm separating said housinginto a valve opening chamber and a valve closing chamber and a linkageconnecting said diaphragm to said air valve for urging said air valvetoward said open position when the pressure in one of said opening andclosing chambers increases relative to the pressure in the other of saidchambers and allowing said spring to close said air valve when thepressure in one chamber decreases relative to the pressure in the other;g. an air passage for communicating atmospheric pressure to said one ofsaid chambers; h. a vacuum passage for communicating the pressure insaid conduit intermediate said air valve and said throttle valve to theother of said chambers; I. a vehicle speed responsive motor control forcontrolling said fluid motor to increase the pressure in said otherchamber as vehicle speed increases above a predetermined speed, saidcontrol including a solenoid having a valve armature element foralternately connecting said air passage and said vacuum passage to saidother chamber, said solenoid operable between energized and deenergizedstates for causing said valve armature element to connect said airpassage to the other chamber when in said deenergized state and toconnect the vacuum passage to the other chamber when in said energizedstate, and vehicle speed responsive switching circuit means including aswitch for alternately energizing and deenergizing said solenoid toswitch said solenoid between said states, said circuit means furtherincluding a switch control mechanism responsive to vehicle speed tocontrol the opening and closing of said switch to increase the durationof said deenergized state relative to the duration of the energizedstate with increasing vehicle speed past said predetermined speed toeffect a smooth transition of vehicle speed to said predetermined limit.